The present invention relates to a soundproofing assembly principally intended for soundproofing of substantially enclosed spaces, such as the passenger compartment of an automobile, application of a foam of plastics material with high tortuosity to such soundproofing, and a method of manufacture of such a soundproofing assembly.
The invention relates to acoustic problems which arise in a substantially enclosed space, such as the passenger compartment of an automobile, in the vicinity of sources of noise such as an engine, the contact of tyres on a road, etc.
Before the description of the prior art it is appropriate to define some terms which are useful for understanding the invention.
In general, in the low frequency range, sound waves undergo a “damping” by materials in the form of single or double sheets (sandwich) or by an effect of porosity and resilience of a mass-spring system, especially using viscoelastic foam.
A soundproofing system ensures an “insulation” when it prevents the entry of medium and high frequency sound waves into the soundproofed space, principally by reflection of the waves towards the noise sources or the exterior of the soundproofed space.
A soundproofing system functions by “sound absorption” (in the range of medium and high frequencies) when the energy of the sound waves is dissipated in an absorbent material.
The invention basically relates to soundproofing in the medium and high frequency range. In general it is relatively simple to obtain good soundproofing at high frequencies with simple means but in the medium frequency range (above all between 400 and 1000 Hz) the problem of soundproofing is all the more acute since the human ear is very sensitive in this frequency range.
Naturally, the different phenomena mentioned above, that is to say damping, insulation and absorption, rarely exist in isolation and they are more often present to varying degrees. Thus the noise in an enclosed space which does not contain sources of noise is the result of contributions from all the sound sources, especially from the engine of an automobile, and from the filtering due to the different actions of the soundproofing material present. Consequently, the soundproofing of an enclosed space is the result of numerous effects, and it is desirable to obtain a good compromise between the soundproofing obtained and the various means implemented for obtaining this soundproofing. For example, it is not necessary to ensure substantial absorption of certain waves in the enclosed space if these have already been practically eliminated by the insulation.